1. Field of the Invention
This invention relates generally to industrial, mining and construction tools, and more specifically to improvements in rotary drag bits and the like for boring, drilling and coring operations.
As used in the following disclosure and claims, the term "polycrystalline diamond" and its abbreviation "PCD" refers to a material formed of individual diamond crystals fused or sintered by intercrystalline bonding under high pressure and temperature into a predetermined layer or shape. The PCD material is usually permanently bonded to a substrate of tungsten carbide in a cobalt binder or like carbide matrix, also known in the art as "precemented carbide". Also, as used herein, the term "high density ceramic" or its abbreviation "HDC" refers to a mining tool having an insert embodying a PCD layer.
2. Prior Art
In the past rotary drilling and coring tools, as used in mining and construction, have been constructed with hardened drill bit cutting heads, and traditionally with sintered carbide inserts to prolong the operative life of the tool. Typical cutting tools may use a single or continuous cutting surface or edge, but frequently employ a plurality of discrete cutting elements or bits either sequentially and angularly arranged on a wheel, caisson or other continuous carrier or otherwise disposed in a predetermined sequence or pattern on a rotary bit or auger of some type. A typical class of heavy duty cutting tools, to which the present invention is particularly applicable involves industrial mining and construction equipment of rotary drag type. This class includes rotary roof bits, longwall radial bits, auger drill bits, undercutter bits, core barrel bits, face drill bits, and two-wing, three-wing and four-wing rotary drag bits--all of which are readily identifiable to those in the mining field.
A principal problem encountered in all of these prior art tools is the rapid wear and high cost of replacement along with machine down-time. This rapid tool wear and breakage, in part due to higher speed equipment and heavier impact forces and tensile stress, has led toward tool redesign with some larger, carbide insert or drilling tip configurations--which in turn has generally resulted in higher dust levels and increased potential ignition dangers contrary to mining safety regulations.
It is believed that a primary and inherent contributing factor in tool wear and breakage heretofore has been the conventional design configuration of such tool bits. Typically, substantially all prior tools have been constructed with a positive to zero rake angle thereby presenting a leading cutting edge point and trailing face that operate with a plow-type action and being subjected to high-point shear forces and tensile stress and drag. The typical positive angularity of cutting edge/face design produces rapid wear and failure, even in the tougher bits using tungsten carbide inserts and the like.
More recently, some substantial advances have been made in harder, tougher compositions for bit inserts. U.S. Pat. No(s). 4,525,178; 4,570,726; 4,604,106 and 4,694,918 disclose some of the basic underlying technology pertaining to such compositions and methods of making PCD materials proposed for use in various oil field drilling and mining operations as well as other machining operations. In particular, U.S. Pat. No. 4,570,726 discloses special insert shapes for drag-type rotary drill bits and suggests a tool having a working surface positioned at a slight negative angle from the perpendicular with respect to the material contacted. In fact, the '726 patent teaches away from the planar-type of working surfaces of both the prior art and the present invention, and discloses specially designed curved face insert configurations for obviating the backup or build-up of loosened material against the working surface. Another patent--U.S. Pat. No. 4,303,136 shows a series of drag bits having diamond surface layers carried on tungsten carbide bodies at a substantial negative rake angle, but this patent relates primarily to the orientation of the working face to hydraulic fluid passages for carrying off the loosened material.